The 16-day shuttle mission includes five space walks, and astronauts will install the last piece of hardware needed to complete Japan's shirtsleeve laboratory, Kibo.

The mission, currently set for a launch at 5:40 a.m., will also turn the space station into an on-orbit version of the Tokyo crowded metro. In addition to the station's six-member crew, the shuttle's arrival will boost the on-orbit population to a record 13 in one place at one time.

Originally, the shuttle was set for launch last Saturday and LRO set to launch Wednesday. But controllers detected a potentially explosive hydrogen leak during Endeavour's fueling Friday evening. Assuming a successful repair, the option was either to launch Endeavour this week or wait until July. NASA opted to launch the shuttle Wednesday, bumping the LRO launch to Thursday.

For shuttle-mission managers, the hydrogen leak was an uncomfortably familiar problem, one that also delayed the launch of the shuttle Discovery in March.

The leak involves a fixture on the outside of the external fuel tank that is vital to a safe fill-up. As ground crews pump liquid hydrogen into the external tank, some of it vaporizes. As the hydrogen tank fills, it pushing the hydrogen gas toward the top. When crews top off the tank, a hose mated to the fixture bleeds off this unwanted gas to keep pressure inside from weakening the tank. The hose routes the gas toward a distant tower, where it's burned.

Similar leaks had occurred in the past. Controllers were able to stop them by repeatedly opening and closing a shut-off valve in the fixture. But a similar leak on the shuttle Discovery failed to respond to this approach.

Engineers have yet to figure out why the two hose connections in the last three flights have leaked. But in March, Discovery's tank leak vanished after technicians replaced seals in the fixture.

NASA officials figured the same strategy would work again. "Our plan is pretty much as it was last time," said Michael Moses, who heads the mission management team, during a recent briefing. Technicians will replace the suspect seal but no one knows if that will solve the problem until the tank is refilled. That should begin tomorrow evening. "Then we'll see what happens," Mr. Moses said.

If the replacement seal fails, the launch could get pushed into July, given the packed schedule for other launches from Cape Canaveral. Wednesday was to have seen the launch of NASA's Lunar Reconnaissance Orbiter and its LCROSS experiment – a part of the LRO's launch vehicle that's designed to slam into the lunar surface to hunt for water ice. The lunar mission is designed to provide information about potential landing sites for humans on the moon, about available resources, and about hazards such as solar and cosmic radiation at potential landing spots.

But the orbiter's mission managers yielded their launch date to Endeavour. The orbiter is now scheduled to launch on June 18, with the first of three opportunities that day coming at 5:12 p.m.